Some things to try:
1. Pinless peepers in the aggressors for two weeks and retry without. If continual bullying some have reported doing longer than two weeks.
2. Vapor rub or something similarly strong on feathers where they are picking. It tastes bad and can curb the behavior though maybe not permanently.
3. Depending on if it is one or two offenders, you can put them in to timeout in a crate or separate them for a couple of weeks. Supposedly this resets flock dynamics.
I'm not keen on bullies. I know there's flock dynamics but if a chicken was going to kill another chicken, then the aggressor would be out. Unless there's a really good reason, like the older chicken is going to die soon, or the aggressor is an integral part of a breeding program or the most consistent egg layer. If there wasn't a compelling point, the biggest bully would be gone. And then I would evaluate if the second worst bully needed to go too.
I am a new chicken keeper though so take all of this with a big grain of salt.